In Denver, Colorado, a gunman used a rifle to fire at least 100 rounds Saturday in an “ambush-style attack” at police who were responding to a disturbance. The suspect, Iraq veteran Matthew Riehl, shot and killed Sheriff Deputy Zackari Parrish and injured four other officers as well as two civilians. The shooting occurred on the final day of a year that saw more than 300 mass shootings, including the deadliest in modern U.S. history in Las Vegas, which left 58 dead and more than 500 injured.